If all else fails, I would recommend performing a Factory Data Rest on your phone. That is found in Settings > Privacy > Factory Data Reset.
You should not need to set USB debugging mode on your device in order to have the MicroSD recognized as a storage device.
When the phone is connected to my PC (either in Windows or Linux - I don't own a Mac), a standard USB icon (looks like a three-fingered fork) appears on my Notification Toolbar at the top of the screen. I then drag down the Notification Toolbar (like a window shade) and tap the USB icon. A message pops up asking if I want to 'mount' (a *NIX term for connect) my device as a storage medium. I tell it to Mount and it then appears as a drive on my computer. In Windows, it will appear as a drive letter under Removable Devices. In Ubuntu Linux, the distro I use, it appears as a drive on my desktop, showing the memory card's capacity (in my case 16 GB Media). Mac users, YMMV. In both Windows and in Linux, if you want your device to perform correctly, you must also 'unmount the drive when you are going to disconnect it. In Windows, this is done by either opening the Safely Remove Hardware applet in the Windows Task Tray, or by right-clicking the device in My Computer (just computer on Win Vista and 7) and unmounting it there. In Ubuntu, right-click the device icon and select Unmount Volume.
Samsung r880 Acclaim/i500 Mesmerize
If I've helped you, please give me thanks. I'll do the same for you!